Inductor core, electronic pen core portion, electronic pen, and input device

ABSTRACT

An inductor core includes: a tubular magnetic material body formed of magnetic material. The tubular magnetic material body includes an inclined portion including an inclined surface which constitutes a peripheral surface of a truncated cone having an outer diameter that increases from one end toward the other end of the tubular magnetic material body; and a straight trunk portion which is disposed coaxially with the inclined portion, includes an outer peripheral surface which constitutes a peripheral surface of a cylindrical body, and is connected with the inclined portion. An arithmetic mean roughness Ra of a region in the outer peripheral surface of the straight trunk portion which region is in a vicinity of the other end, is smaller than an arithmetic mean roughness Ra of a region in the outer peripheral surface of the straight trunk portion which region is in a vicinity of the inclined portion.

TECHNICAL FIELD

The present invention relates to an inductor core, an electronic pen core portion, an electronic pen, and an input device.

BACKGROUND ART

For an input device that detects a position in a tablet or a display provided with a position detection sensor and inputs position information to a personal computer (PC), a smartphone, and the like, an electronic pen is used to indicate the position on the position detection sensor.

Detection between the position detection sensor and the electronic pen is enabled with the position detection device, through transmission and reception of a position detection signal by a coupling method such as an electromagnetic inductive coupling method or an electrostatic inductive coupling method (for example, Patent Literature 1).

The electronic pen used in such an input device includes an inductor core configured by disposing a magnetic material such as ferrite around a core member of the electronic pen.

CITATION LIST Patent Literature

Patent Literature 1: WO 2017/183526 A1

SUMMARY OF INVENTION Solution to Problem

An inductor core according to the present disclosure includes a tubular magnetic material body formed of magnetic material, the tubular magnetic material body including an inclined portion including an inclined surface which constitutes a peripheral surface of a truncated cone having an outer diameter that increases from one end toward the other end of the tubular magnetic material body, and a straight trunk portion which is disposed coaxially with the inclined portion, includes the straight trunk portion including an outer peripheral surface which constitutes a peripheral surface of a cylindrical body extending from the other end toward the one end of the tubular magnetic material body, and is connected with the inclined portion, an arithmetic mean roughness Ra of a region in the outer peripheral surface of the straight trunk portion which region is in a vicinity of the other end being smaller than an arithmetic mean roughness Ra of a region in the outer peripheral surface of the straight trunk portion which region is in a vicinity of the inclined portion.

An electronic pen core portion according to the disclosure includes the inductor core described above, and a core member which is inserted into the inductor core such that a tip portion of the core member protrudes from one end of the inductor core.

An electronic pen according to the disclosure includes a housing provided with an opening, and the electronic pen core portion described above, the electronic pen core portion being housed in the housing such that the tip portion of the electronic pen core portion can protrude or protrudes from the opening of the housing.

An input device according to the disclosure includes the electronic pen described above, and a position detection device including a sensor that detects a position which the electronic pen is approaching.

BRIEF DESCRIPTION OF DRAWINGS

Other and further objects, features, and advantages of the invention will be more explicit from the following detailed description taken with reference to the drawings wherein:

FIG. 1 is a plan view showing an example of an inductor core according to a first embodiment;

FIG. 2 is a partial cross-sectional view showing an example of the inductor core according to the first embodiment;

FIG. 3 is a plan view showing an example of an inductor core according to a second embodiment;

FIG. 4 is a plan view showing an example of an electronic pen core portion according to an embodiment;

FIG. 5 is a plan view showing an example of an electronic pen according to an embodiment;

FIG. 6 is a perspective view showing an example of an input device according to an embodiment; and

FIG. 7 is a photograph showing an example of an observation surface of an inductor core and how straight lines are drawn.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an inductor core according to the disclosure will be described in detail with reference to the drawings.

FIG. 1 is a plan view showing an example of the inductor core according to a first embodiment. An inductor core 1 includes a magnetic material body 10 formed of a magnetic material such as ferrite sintered body.

The inductor core 1 is formed of a tubular magnetic material body 10 and includes a cylindrical hole 10 c penetrating from one end 10 a to the other end 10 b. The magnetic material body 10 includes an inclined portion 11 including an inclined surface 11 a which constitutes a peripheral surface of a truncated cone having an outer diameter that increases from the one end 10 a toward the other end 10 b, and a straight trunk portion 12 which is disposed coaxially with the inclined portion 11, includes an outer peripheral surface 12 a of a cylindrical body extending from the other end 10 b toward the one end 10 a, and is connected with the inclined portion 11.

The length from the one end 10 a to the other end 10 b of the magnetic material body 10 is about 5 mm to 15 mm, and the diameter of the cylindrical hole 10 c is about 0.5 mm to 2.0 mm, for example. The length of the straight trunk portion 12 is about 3 mm to 12 mm, and the outer diameter of the straight trunk portion 12 is about 2.0 mm to 3.0 mm. The length of the inclined portion 11 is about 0.5 mm to 2.0 mm, and the outer diameter of the inclined portion 11 on the side of the one end 10 a is about 1 mm to 2 mm, and the outer diameter of the inclined portion 11 at the opposite side to the one end 10 a is substantially the same as the outer diameter of the straight trunk portion 12. As described above, the inclined portion 11 is tapered toward the one end 10 a.

In the cross section along a central axis of the magnetic material body 10, the outer diameter of the inclined portion 11 increases from the one end 10 a toward the other end 10 b. That is, the inclined portion 11 is tapered toward the one end 10 a. The inclined surface 11 a may include an inclined surface 11 a 1 which is a linear portion in a cross-sectional view, and a rounded inclined surface 11 a 2. The inclined surface 11 a 1 may form a peripheral surface of a truncated cone. The inclined surface 11 a in the vicinity of the one end 10 a may be a convex curved surface. That is, the inclined surface 11 a 1 of the inclined portion 11 and an end surface 11 b of the inclined portion 11 may be connected to each other by the inclined surface 11 a 2 that is a part of the inclined surface 11 a and that is a curved surface in convex shape.

As described above, since the inclined surface 11 a 1 and the end surface 11 b are connected to each other by the inclined surface 11 a 2 which is the convex curved surface, the possibility of damage may be reduced when, for example, the end surface 11 b of the inclined portion 11 comes into contact with the housing of the electronic pen. When the electronic pen is tilted and brought into contact with the surface of a tablet or the like, in addition to the core member, the tip including the one end 10 a of the magnetic material body 10 may also come into contact with the tablet or the like, but because the inclined surface 11 a 1 and the end surface 11 b of the inclined portion 11 are connected by the inclined surface 11 a 2 which is the convex curved surface, there is no presence of a pointed portion, and it is thus possible to reduce the possibility of damaging the surface of the tablet or the like by the inductor core 1.

In an inner peripheral surface 10 e of the cylindrical hole 10 c of the magnetic material body 10, the inner peripheral surface 10 e near an opening 10 d at the inclined portion 11 side may be rounded. The inner peripheral surface 10 e may include an inner peripheral surface 10 e 1 and an inner peripheral surface 10 e 2 that is a convex curved surface and that connects the inner peripheral surface 10 e 1 and the end surface 11 b of the inclined portion 11. When the inner surface of the cylindrical hole 10 c and the end surface 11 b of the inclined portion 11 are connected to each other by the inner peripheral surface 10 e 2 which is a convex curved surface, because the concentration of stress may be reduced and the possibility of damaging the magnetic material body 10 may be reduced when, for example, the core member is pressed against the surface of the tablet or the like, the inductor core 1 having high reliability may be realized.

In the cross section along the central axis of the magnetic material body 10, when the radius of curvature of the inclined surface 11 a 2 which connects the inclined surface 11 a 1 of the inclined portion 11 and the end surface 11 b of the inclined portion 11 is R1, and the radius of curvature of the inner peripheral surface 10 e 2 which connects the inner peripheral surface 10 e 1 of the cylindrical hole 10 c and the end surface 11 b of the inclined portion 11 is R2, the radius of curvature R1 of the inclined surface 11 a 2 may be greater than the radius of curvature R2 of the inner peripheral surface 10 e 2. The radius of curvature R1 of the inclined surface 11 a 2 is 0.1 mm to 0.2 mm, for example, and the radius of curvature R2 of the inner peripheral surface 10 e 2 is 0.02 mm to 0.08 mm, for example.

FIG. 3 is a plan view showing an example of an inductor core according to a second embodiment. There is a difference from the inductor core 1 of the first embodiment in that the straight trunk portion 12 and the inclined portion 11 are connected to each other with the flange portion 13. The magnetic material body 10 is configured with the inclined portion 11 including the inclined surface 11 a which constitutes a peripheral surface of the truncated cone having the outer diameter that increases from the one end 10 a toward the other end 10 b, the straight trunk portion 12 which is disposed coaxially with the inclined portion 11, and constitutes the outer peripheral surface 12 a of the cylindrical body that extends from the other end 10 b toward the one end 10 a, and the flange portion 13 which is provided between the inclined portion 11 and the straight trunk portion 12 and connects the inclined portion 11 and the straight trunk portion 12. The inclined portion 11, the flange portion 13 and the straight trunk portion 12 are arranged in this order from the one end 10 a toward the other end 10 b, and an outer peripheral surface 13 a of the flange portion 13 has a greater outer diameter than the outer diameter of each of the inclined surface 11 a of the inclined portion 11 and the outer peripheral surface 12 a of the straight trunk portion 12.

For example, when the outer diameter of the straight trunk portion 12 other than the flange portion 13 is 2.1 mm to 2.5 mm, and the maximum value of the outer diameter of the outer peripheral surface 13 a of the flange portion 13 is 2.12 mm to 2.72 mm, the flange portion 13 protrudes from the outer peripheral surface 12 a of the straight trunk portion 12 by a maximum of 0.02 mm to 0.22 mm. Since the straight trunk portion 12 is less likely to be deformed by the flange portion 13, even when a force is applied to the inclined portion 11 and the inclined portion 11 is deformed, the deformation of the straight trunk portion 12 may be reduced.

The inductor core 1 illustrated in the first embodiment and the second embodiment as described above is used by inserting a core member described below into the cylindrical hole 10 c. The core member is inserted such that the tip portion of the core member is on the side of the one end 10 a of the magnetic material body 10. Since the inclined portion 11 has a tapered shape, the one end 10 a of the magnetic material body 10 may be brought closer to a position detection device such as a tablet that detects the position by electromagnetic induction method or the like. As described above, by forming the inclined portion 11 of the magnetic material body 10 in a tapered shape, the accuracy of the position detection of the position detection device is improved. Since the tip portion of the magnetic material body 10 is tapered, the inclined portion 11 is configured to be easily displaced when the core member is displaced by the force applied to the tip portion of the core member.

An arithmetic mean roughness Ra of a region in the outer peripheral surface 12 c of the straight trunk portion 12 which region is in the vicinity of the other end 10 b is smaller than an arithmetic mean roughness Ra of a region in the outer peripheral surface 12 b of the straight trunk portion 12 which region is in the vicinity of the inclined portion 11. As a result, when a coil is formed by winding a coated wire such as a litz wire on the outer peripheral surface 12 c of the straight trunk portion 12 in the vicinity of the other end 10 b, it is possible to reduce the possibility that the coil is disconnected or the coating of the coil is damaged.

A root mean square roughness Rq of the region in the outer peripheral surface 12 c of the straight trunk portion 12 which region is in the vicinity of the other end 10 b is smaller than a root mean square roughness Rq of the region in the outer peripheral surface 12 b of the straight trunk portion 12 which region is in the vicinity of the inclined portion 11. As a result, when a coil is formed by winding a coated wire such as a litz wire on the outer peripheral surface 12 c of the straight trunk portion 12 in the vicinity of the other end 10 b, it is possible to reduce the possibility that the coil is disconnected or the coating of the coil is damaged.

A section height difference (Rδc) of the outer peripheral surface 12 c of the straight trunk portion 12 in the vicinity of the other end 10 b is smaller than a section height difference (Rδc) of the outer peripheral surface 12 b of the straight trunk portion 12 in the vicinity of the inclined portion 11. As a result, when a coil is formed by winding a coated wire such as a litz wire on the outer peripheral surface 12 c of the straight trunk portion 12 in the vicinity of the other end 10 b, it is possible to reduce the possibility that the coil is disconnected or the coating of the coil is damaged.

In the present example, the section height difference (Rδc) between the outer peripheral surfaces 12 c and 12 b is the difference between a section height at a material ratio of 25% in the roughness profile of the target outer peripheral surface and a section height at a material ratio of 75% in the roughness profile of that outer peripheral surface.

Table 1 shows the measurement results of the surface roughness of the magnetic material body 10 forming the inductor core 1 according to the disclosure described above. The arithmetic mean roughness Ra and the root mean square roughness Rq of the roughness profile were measured in accordance with JIS B 0601:2001, using a laser microscope (manufactured by Keyence Corp., ultra-depth color 3D shape measuring microscope (VK-9500)). The measurement conditions were as follows: 200× magnification (eyepiece: 20×, objective lens: 10×), the measurement mode was color super-depth, the measurement resolution (pitch) in the height direction was 0.05 μm, the optical zoom was 1.0 times, the gain was 953, the observation range was 1315 μm×982 μm, the cutoff value λs was 2.5 μm, and the cutoff value λc was 0.08 mm. In the observation range, a range in focus (for example, 815 μm to 1315 μm×425 μm to 625 μm) was selected as the measurement range. To calculate the numerical values, with respect to 8 samples or more, the average value of the obtained values was calculated as the arithmetic mean roughness Ra, the root mean square roughness Rq and the section height difference Rδc of the roughness profile in the outer peripheral surfaces of respective parts.

TABLE 1 Measurement location Ra(μm) Rq(μm) Rδc(μm) Inclined surface of inclined portion 6.88 11.68 7.18 Outer peripheral surface of straight 4.96 8.36 5.43 trunk portion (in the vicinity of inclined portion) Outer peripheral surface of straight 3.76 6.22 4.50 trunk portion (in the vicinity of the other end)

As shown in Table 1, the arithmetic mean roughness Ra is 6.88 μm in the inclined surface 11 a of the inclined portion 11, 4.96 μm in the outer peripheral surface 12 b of the straight trunk portion 12 in the vicinity of the inclined portion 11, and 3.76 μm in the outer peripheral surface 12 c of the straight trunk portion 12 in the vicinity of the other end 10 b. As described above, the arithmetic mean roughness Ra of the region in the outer peripheral surface 12 c of the straight trunk portion 12 which region is in the vicinity of the other end 10 b is smaller than the arithmetic mean roughness Ra of the region in the outer peripheral surface 12 b of the straight trunk portion 12 which region is in the vicinity of the inclined portion 11.

The root mean square roughness Rq is 11.68 μm in the inclined surface 11 a of the inclined portion 11, 8.36 μm in the outer peripheral surface 12 b of the straight trunk portion 12 in the vicinity of the inclined portion 11, and 6.22 μm in the outer peripheral surface 12 c of the straight trunk portion 12 in the vicinity of the other end 10 b. As described above, a root mean square roughness Rq of the region in the outer peripheral surface 12 c of the straight trunk portion 12 which region is in the vicinity of the other end is smaller than a root mean square roughness Rq of the region in the outer peripheral surface 12 b of the straight trunk portion 12 which region is in the vicinity of the inclined portion 11.

A section height difference Rδc is 7.18 μm in the inclined surface 11 a of the inclined portion 11, 5.43 μm in the outer peripheral surface 12 b of the straight trunk portion 12 in the vicinity of the inclined portion 11, and 4.50 μm in the outer peripheral surface 12 c of the straight trunk portion 12 in the vicinity of the other end 10 b. As described above, a section height difference Rδc in the outer peripheral surface 12 c of the straight trunk portion 12 in the vicinity of the other end is smaller than a section height difference Rδc in the outer peripheral surface 12 b of the straight trunk portion 12 in the vicinity of the inclined portion 11.

The arithmetic mean roughness Ra of the region in the outer peripheral surface 12 b of the straight trunk portion 12 which region is in the vicinity of the inclined portion 11 is set to 6 μm or less, resulting in the arithmetic mean roughness Ra of the region in the outer peripheral surface 12 c in the vicinity of the other end 10 b, around which the coil is wound, being further lowered, and accordingly, when the coil is arranged by winding a coated wire such as a litz wire on the outer peripheral surface 12 c of the straight trunk portion 12 in the vicinity of the other end 10 b, it is possible to reduce the possibility that the coil is disconnected or the coating of the coil is damaged.

The root mean square roughness Rq of the region in the outer peripheral surface 12 b of the straight trunk portion 12 which region is in the vicinity of the inclined portion 11 is set to 10 μm or less, resulting in the arithmetic mean roughness of the region in the outer peripheral surface 12 c which region is in the vicinity of the other end 10 b, around which the coil is wound, being further lowered, and accordingly, when the coil is formed by winding a coated wire such as a litz wire on the outer peripheral surface 12 c of the straight trunk portion 12 in the vicinity of the other end 10 b, it is possible to reduce the possibility that the coil is disconnected or the coating of the coil is damaged.

The magnetic material body 10 is formed of ceramics which includes oxides of Fe, Zn, Ni, and Cu, and contains ferrite as a main component, and a coefficient of variation CV of average crystal grain size of the ceramics expressed by the following formula (1) may be 0.08 or more and 0.3 or less:

CV=σ/x  (1)

where,

x is an average value of the average crystal grain sizes of the ceramic, and

σ is a standard deviation of the average crystal grain sizes of the ceramic.

When the coefficient of variation CV is 0.08 or more, the grain size of the crystal grains varies appropriately, and small crystal grains are arranged among large crystal grains, thereby increasing the fracture toughness. When the coefficient of variation CV is 0.3 or less, the ratio of crystal grains having a large grain size with respect to the standard deviation increases, and the magnetic permeability is thereby increased. When the coefficient of variation CV is 0.08 or more and 0.3 or less, both high fracture toughness and high permeability may be achieved.

In particular, the coefficient of variation CV is preferably 0.1 or more and 0.2 or less.

In the example, the average crystal grain size may be obtained as follows.

First, the fractured surface of the inductor core 1 is polished with a copper disk using diamond abrasive grains having an average particle diameter D₅₀ of 3 μm, and then is ground on a tin disk using diamond abrasive grains having an average particle diameter D₅₀ of 0.5 μm. The polished surface obtained by these polishing is etched at a temperature of 950° C. until the crystal particles and the grain boundary layer may be distinguished from each other so that an observation surface is obtained.

Within a range of 155 μm×115 μm of the observation surface at 5000× magnification with a scanning electron microscope, six straight lines of the same length of 100 μm, for example, are drawn around any point radially, and the lengths of these six straight lines are divided by the number of crystals present on each line, so that the average crystal grain size can be obtained.

FIG. 7 is a photograph showing an example of the observation surface of the inductor core and how the straight lines are drawn. The straight lines A to F shown in FIG. 7 are straight lines each having a length of 100 μm, and the average crystal grain size may be obtained using the straight lines. The average value, standard deviation, and coefficient of variation CV of the average crystal grain sizes may be calculated for each of 42 average crystal grain sizes by selecting 7 such observation surfaces.

The kurtosis Ku of the average crystal grain sizes may be 0 or more.

When the kurtosis Ku of the average crystal grain sizes is within the above range, since the variation of the grain size of the crystal grain is reduced, the agglomeration of the pores is reduced, and it is thus possible to reduce the degranulation from the contour or the inside of the pores. Particularly, the kurtosis Ku of the average crystal grain sizes is preferably 1 or more.

Here, the kurtosis Ku is an index (statistic) indicating how much the peak and tail of the distribution differ in the normal distribution, in which when the kurtosis Ku>0, the distribution has a sharp peak, when the kurtosis Ku=0, the distribution is normal, and when the kurtosis Ku<0, the distribution has a rounded peak.

The kurtosis Ku of the average crystal grain sizes may be obtained by using the function Kurt provided in Excel (registered trademark, Microsoft Corporation).

The skewness Sk of the average crystal grain sizes may be 0 or more.

When the skewness Sk of the average crystal grain sizes is within the above range, since the distribution of the grain size of the crystal grains is moving in the direction of the smaller grain size, the agglomeration of the pores is reduced, and it is thus possible to further reduce the degranulation from the contour or the inside of the pores.

Here, the skewness Sk is an index (statistic) indicating how much the distribution is distorted from the normal distribution, that is, the left-right symmetry of the distribution, in which, when the skewness Sk>0, the tail of the distribution moves to the right side, when the skewness Sk=0, the distribution is left-right symmetrical, and when the skewness Sk<0, the tail of the distribution is moved to the left side.

The skewness Sk of the average crystal grain sizes may be obtained by using the function SKEW provided in Excel (registered trademark, Microsoft Corporation).

At least the inclined portion 11 may contain Mo, which is contained more in a grain boundary layer than in crystal grains.

When Mo is contained more in the grain boundary layer than the crystal grains, since the binding force between the crystal grains containing ferrite as a main component is reduced, the inclined surface 11 a 2 having a large radius of curvature R1 may be easily obtained.

The Mo content in the crystal grains and in the grain boundary layer may be subjected to elemental analysis by using a transmission electron microscope and an energy dispersive X-ray spectrometer (EDS) attached to the transmission electron microscope.

The magnetic material body 10 used for the inductor core 1 may be manufactured as follows. First, as starting materials, oxides of Fe, Zn, Ni and Cu, or metal salts such as carbonates and nitrates for producing oxides by firing are prepared. Here, the average particle diameter is 0.5 μm or more and 5 μm or less, respectively, when Fe is iron oxide (Fe₂O₃), Zn is zinc oxide (Zno), Ni is nickel oxide (NiO) and Cu is copper oxide (CuO), for example.

Subsequently, in producing a first raw material formed of a calcined powder containing Fe₂O₃—ZnO—NiO and a second raw material formed of a calcined powder containing Fe₂O₃—CuO, iron oxide, zinc oxide, and nickel oxide are weighed in desired amounts for the first raw material. For the second raw material, iron oxide and copper oxide are weighed in desired amounts. In the example, regarding the addition amount of iron oxide in the production of the first raw material and the second raw material, the amount of iron oxide added in the production of the second raw material is equimolar in percentage to that of copper oxide, and the remaining amount is used in the production of the first raw material.

Then, the powders weighed for the first raw material and the second raw material are pulverized and mixed with different ball mills or vibration mills, respectively, and then are calcined in a reducing atmosphere at 750° C. for 2 hours or longer in the production of the first raw material and in a reducing atmosphere at 650° C. for 2 hours or longer in the production of the second raw material, respectively to obtain a calcined body, respectively.

Next, the calcined bodies to be the first raw material and the second raw material are put into separate ball mills or vibration mills and pulverized to obtain the first raw material and the second raw material formed of the calcined powder. Here, particularly, the calcined body as the second raw material is pulverized so that the average particle diameter D50 is 0.7 μm or less. Then, the first raw material and the second raw material are weighed and mixed in a desired amount, and then recalcined in the atmosphere under the conditions of 600° C. or higher and 700° C. or lower and a heating rate of 100° C./h or lower to obtain a calcined body synthesized into ferrite formed of oxides of Fe, Zn, Ni and Cu.

Next, the calcined body obtained by the re-calcination is put into a ball mill, a vibration mill, or the like and pulverized, is made into a slurry by adding a predetermined amount of binder or the like, and granulated by spraying the slurry using a spray dryer to obtain spherical granules.

Here, when obtaining the inductor core 1 in which at least the inclined portion 11 contains Mo, which is contained more in the grain boundary layer than in the crystal grains, with respect to 100 parts by mass of the calcined body obtained by the re-calcination, for example, 0.01 part by mass or more and 0.03 part by mass or less of a powder of molybdenum oxide (MoO₃) is added to form the slurry, and the slurry may be sprayed for granulation to obtain spherical granules.

Then, the obtained spherical granules are press-molded to obtain a molded body having a predetermined shape. At the time of the press molding, when adjusting the surface texture of the molding die, by adjusting such that a greater pressure is applied to the straight trunk portion 12 side than to the inclined portion 11 side, the inclined surface 11 a and the outer peripheral surface 12 a may be configured such that the arithmetic mean roughness Ra and the root mean square roughness Rq decreases from the one end 10 a toward the other end 10 b of the magnetic material body 10, and the values of the surface roughness Ra, and Rq of the outer peripheral surface 12 a of the straight trunk portion 12 may be made smaller than that of the inclined surface 11 a. After that, the molded body is subjected to a degreasing treatment in a degreasing furnace in the range of 400 to 800° C. to obtain a degreased body, which is then held and fired in a firing furnace at a maximum temperature of 1000 to 1200° C. for 2 to 5 hours to form the magnetic material body 10, so that the inductor core 1 according to the present embodiment may be obtained.

Since the magnetic material body 10 forming the inductor core 1 according to the first and second embodiments is formed by press molding, there is no need to perform a process such as cutting, and thus the cost may be reduced. When the post-processing such as cutting is performed, the surface may be deteriorated by the post-processing to impair the characteristics of the inductor core, but in the present embodiment, the inductor core 1 is not processed after firing and the surface deterioration is prevented. As a result, the inductor core 1 having high reliability may be formed.

FIG. 4 is a plan view showing an example of an electronic pen core portion according to an embodiment. The electronic pen core portion 2 includes the inductor core 1, a coil 21 wound around the magnetic material body 10 of the inductor core 1, and a core member 22 inserted into the cylindrical hole 10 c of the magnetic material body 10. Such an electronic pen core portion 2 may be built in the electronic pen of the input device such as an electromagnetic induction type tablet.

The core member 22 may be formed of a metal rod such as SUS304 or SUS316 that is not likely to be magnetized, metal material other than SUS, ceramics, or resin. The core member 22 may be one that is capable of actually writing, such as a ballpoint pen core. The core member 22 is inserted into the cylindrical hole 10 c of the magnetic material body 10 and fixed therein. The core member 22 is fixed in the magnetic material body 10 at a position such that the tip portion 22 a protrudes from the opening 10 d on the one end 10 a side of the magnetic material body 10 by about 1 to 2 mm. A rear end portion 22 b of the core member 22 protrudes from an opening 10 f on the other end 10 b of the magnetic material body 10.

The coil 21 formed by winding an enamel wire or the like is arranged on the outer peripheral surface 12 a of a region of the straight trunk portion 12 of the magnetic material body 10, which is close to the other end 10 b. The coil 21 is wound with a width of about 8 mm to 12 mm around a portion of the straight trunk portion 12 of the magnetic material body 10, which is close to the other end side. The terminals 21 a and 21 b of the coil 21 are connected to a circuit board (not shown).

The coil 21 is arranged by winding a coated wire such as an enamel wire, a litz wire, a USTC wire or a urethane wire around the magnetic material body 10. The coil 21 is arranged on the outer peripheral surface 12 c of the straight trunk portion 12 in the vicinity of the other end 10 b. When the surface roughness, such as the arithmetic mean roughness Ra and the root mean square roughness Rq of the region in the outer peripheral surface 12 c, is large, there is a risk that the coating of the wound coil 21 is damaged, but the arithmetic mean roughness Ra of the region in the outer peripheral surface 12 c of the straight trunk portion 12 which region is in the vicinity of the other end 10 b is made smaller than the arithmetic mean roughness Ra of the region in the outer peripheral surface 12 b of the straight trunk portion 12 which region is in the vicinity of the inclined portion 11, and the coil 21 is disposed on the outer peripheral surface 12 c which is smoother, and accordingly, the coil 21 is less likely to be damaged, and it is possible to realize the electronic pen core portion 2 having high reliability.

FIG. 5 is a view showing an electronic pen according to an embodiment, which shows the electronic pen 3 with a part of the housing 31 thereof removed. The electronic pen core portion 2 is housed in the housing 31 to form the electronic pen 3. The electronic pen 3 is configured to house the electronic pen core portion 2 and the circuit board (not shown) in a hollow portion 31 a of the tubular housing 31. Such an electronic pen 3 may be used as a position input means in the input device such as an electromagnetic induction type tablet. The tip portion 31 b of the housing 31 is provided with an opening 31 c through which the tip portion 22 a of the core member 22 may protrude, and configured such that the tip portion 22 a may protrude from the opening 31 c or may be housed in the housing 31 by a knock mechanism.

For example, the rear end 31 d of the housing 31 is provided with an opening 31 e, and a knock rod 32 protrudes from the opening 31 e. The user may press the knock rod 32 to move the tip portion 22 a of the core member 22 into and out of the housing 31. While the tip portion 22 a of the core member 22 is configured to be moved into and out of the opening 31 c in the present embodiment, the tip portion 22 a of the core member 22 may be fixed in the state of protruding from the opening 31 c, in which case the knock mechanism is unnecessary. Since the electronic pen 3 as described above has built therein the electronic pen core portion 2 including the coil 21 that is less likely to be damaged, the electronic pen 3 having high reliability may be realized.

FIG. 6 is a perspective view showing an input device according to an embodiment. An input device 4 includes the electronic pen 3, and a tablet 41 that is a position detection device including a sensor that detects a position. The input device 4 may detect a position at which the tip portion 22 a of the core member 22 comes into contact with the tablet 41. In addition to the tablet 41, the position detection device may be a mobile terminal equipped with a touch panel display, or the like. The electromagnetic induction may be used as the position detection method for the input device 4. Since the input device as described above includes the electronic pen 3 having the built-in electronic pen core portion 2 including the coil 21 that is less likely to be damaged, the input device 4 having high reliability may be realized.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and the range of equivalency of the claims are therefore intended to be embraced therein.

REFERENCE SIGNS LIST

-   -   1: Inductor core     -   2: Electronic pen core portion     -   3: Electronic pen     -   4: Input device     -   10: Magnetic material body     -   10 a: One end     -   10 b: Other end     -   11: Inclined portion     -   11 a: Inclined surface     -   12: Straight trunk portion     -   12 a, 12 b, 12 c: Outer peripheral surface 

1. An inductor core, comprising: a tubular magnetic material body formed of magnetic material, the tubular magnetic material body comprising an inclined portion comprising an inclined surface which constitutes a peripheral surface of a truncated cone having an outer diameter that increases from one end toward the other end of the tubular magnetic material body; and a straight trunk portion which is disposed coaxially with the inclined portion, comprises an outer peripheral surface which constitutes a peripheral surface of a cylindrical body extending from the other end toward the one end of the tubular magnetic material body, and is connected with the inclined portion, an arithmetic mean roughness Ra of a region in the outer peripheral surface of the straight trunk portion which region is in a vicinity of the other end, being smaller than an arithmetic mean roughness Ra of a region in the outer peripheral surface of the straight trunk portion which region is in a vicinity of the inclined portion.
 2. The inductor core according to claim 1, wherein the arithmetic mean roughness Ra of the region in the outer peripheral surface of the straight trunk portion which region is in the vicinity of the inclined portion is 6 μm or less.
 3. The inductor core according to claim 1, wherein a root mean square roughness Rq of the region in the outer peripheral surface of the straight trunk portion which region is in the vicinity of the other end is smaller than a root mean square roughness Rq of the region in the outer peripheral surface of the straight trunk portion which region is in the vicinity of the inclined portion.
 4. The inductor core according to claim 3, wherein the root mean square roughness Rq of the region in the outer peripheral surface of the straight trunk portion which region is in the vicinity of the inclined portion is 10 μm or less.
 5. The inductor core according to claim 1, wherein the tubular magnetic material body is formed of ceramics which comprises oxides of Fe, Zn, Ni and Cu, and comprises ferrite as a main component, and a coefficient of variation CV of average crystal grain sizes of the ceramics expressed by the following formula (1) is 0.08 or more and 0.3 or less: CV=σ/x  (1) where, x is an average value of the average crystal grain sizes of the ceramics, and σ is a standard deviation of the average crystal grain sizes of the ceramics.
 6. The inductor core according to claim 5, wherein a kurtosis Ku of the average crystal grain sizes is 0 or more.
 7. The inductor core according to claim 5, wherein a skewness Sk of the average crystal grain sizes is 0 or more.
 8. The inductor core according to claim 5, wherein at least the inclined portion comprises Mo, which is contained more in a grain boundary layer than in crystal grains.
 9. An electronic pen core portion, comprising: the inductor core according to claim 1; and a core member which is inserted into the inductor core such that a tip portion of the core member protrudes from one end of the inductor core.
 10. An electronic pen, comprising: a housing provided with an opening; and the electronic pen core portion according to claim 9, the electronic pen core portion being housed in the housing such that the tip portion of the electronic pen core portion can protrude or protrudes from the opening of the housing.
 11. An input device, comprising: the electronic pen according to claim 10; and a position detection device comprising a sensor that detects a position which the electronic pen is approaching. 